1. Technical Field
The present invention relates to a focused ion beam apparatus equipped with a gas field ion source.
2. Description of the Related Art
A sharpened needle-shaped electrode for generating ions in a gas field ion source (GFIS) of a focused ion beam (FIB) apparatus is called a tip.
In focused ion beam apparatuses equipped with a gas field ion source of the related art, the free end of the tip is sharpened to be composed of several atoms to achieve high resolution.
First, the principle of generating ions by a gas field ion source is described with reference to FIG. 10.
A gas to be ionized is supplied into a gas field ion source chamber and gas molecules or atoms 501 (gas molecules in this case) of helium or hydrogen to be ionized exist around a sharpened tip 500. The tip 500 has been cooled by a cooling unit (not shown).
When power is applied between the tip 500 and an extraction electrode 503 by a power 502 and a high electric field is generated around the free end of the tip 500, the gas molecules 501 around the tip 500 are polarized and attracted to the free end of the tip 500 by polarization force. The attracted gas molecules 501 are ionized by the high electric field at the free end of the tip 500. The ions 504 are emitted to a sample (not shown) through an ion-optical system (not shown) under a hole 503a of the extraction electrode 503.
The size of the area through which beams of the ions 504 (ion beams) are emitted, that is the source size (actual ion emission area) of the gas field ion source is very small, so the gas field ion source becomes a ion source having high luminance and thus it is possible to make very thinly focused ion beams on the sample.
FIGS. 11A to 11C show a schematic shape of the tip 500 of the related art. FIG. 11A shows the entire shape of the tip 500. The tip 500 is formed in a thin and sharpened shape by applying electrolytic polishing (also called wet etching) to the free end of a thin wire having a thickness of hundreds of micrometers or less. FIG. 11B is an enlarged view of the free end P of the tip 500, and as shown in FIG. 11B, the tip 500 has a small projection 505 at the free end. The projection 505, as shown in FIG. 11C, has a schematically pyramidal shape formed by several atomic layers and the apex of the projection 505 is composed of plurality of atoms. Ions 504 of gas molecules are emitted from the position of the outermost atom (at the free end) when a focused ion beam is generated. The projection 505 is referred to as a pyramid structure hereafter.
In the related art, there have been known a gas field ion source using a tungsten tip and an ion microscope (also called a focused ion beam apparatus) equipped with the gas field ion source using a tungsten tip. In general, tips are formed by applying electrolytic polishing to a monocrystal material and it has been known that a facet having low atomic density in a tip surface is easily sharpened. A tungsten tip is sharpened in a direction <111>. A {111} facet of tungsten is triple rotationally symmetric and a {110} or {112} facet becomes a pyramidal side, thereby making a triangular pyramidal structure. The apex is stable when it has a structure composed of three atoms (also called a terpolymer) and ions are emitted from the points of the three atoms.
As a method of sharpening the free end of a tungsten tip with several atoms, there are field-induced gas etching that uses nitrogen or oxygen, thermal faceting, and remolding etc., and the tungsten tip can be effectively sharpened in a <111> orientation.
The tip 500 constitutes a gas field ion source by being attached to a tip assembly. FIG. 12 is a perspective view of a tip assembly 506 of the related art.
The tip assembly 506 includes a pair of electric pins 508 fixed to an insulating base 507, a filament 509 made of a thin wire having a high melting point such as tungsten and disposed between the free ends of the pair of electric pins 508, and a needle-shaped tip 500 mechanically and electrically fixed to the filament 509. The tip 500 is fixedly connected to the filament 509, for example, by spot welding so that current can flow between the electric pins 508, so the filament 509 can be heated to high temperature and the heat can be conducted to the tip 500, thereby heating the tip 500. Ions are emitted from the free end of the tip 500. The needle-shaped tip 500 is made of a thin wire having a circular cross-section electrically and mechanically fixed to the filament 509 and the free end of the tip 500 is sharpened at the atomic level by electrolytic polishing. The tip 500 is made of a monocrystal material such as high-purity tungsten or iridium.
FIG. 13 shows the basic configuration of a gas field ion source 510 of the related art.
The gas field ion source 510 includes an extraction electrode 503, a tip assembly 506, an ion source gas supplier 511, a cooling unit 512, and an extraction power (not shown).
The extraction electrode 503 is spaced from the free end of the tip 500 and has a hole 503a. The extraction electrode 503 guides ions 504 emitted from the tip 500 to an ion-optical system at downstream of the hole 503a. 
The extraction power (not shown) can apply extraction power between the extraction electrode 503 and the tip 500, and accordingly, gas molecules 501 are ionized into ions 504 at the free end of the tip 500 and the ions 504 are extracted to the extraction electrode 503.
The ion source gas supplier 511 can supply a small amount of gas (for example, a helium gas) of the gas molecules 501 to be ionized around the tip 500 and is connected to a ion source chamber 513 through a gas supply pipe 511b through which a flow rate can be adjusted by a valve 511a. 
The cooling unit 512 cools the tip 500 and the gas molecules 501 supplied to the ion source chamber 513 from the ion source gas supplier 511, using a refrigerant such as liquid helium or liquid nitrogen. A low-temperature refrigerant produced by the cooling unit 512 comes in contact with walls 514 surrounding the tip assembly 506 and the gas supply pipe 511b through a coupling unit 512a, thereby cooling the inside of the ion supply chamber 513 in addition to them.
Next, a process of manufacturing a tip with a small projection at the free end in the related art is described hereafter. In order to manufacture such a tip, electrolytic polishing, field-induced gas etching, thermal faceting, and remolding etc. have been used in the related art.
The field-induced gas etching is a method of etching a tungsten tip by applying a nitrogen gas while observing a FIM (Field Ion Microscope) image through a field ion microscope using helium as an image forming gas. Nitrogen is smaller in field ionization intensity than helium, so nitrogen gas cannot come close to the area where an FIM image is observed (that is, the area where helium is ionized) and is absorbed to a tip side slightly away from the free end of the tungsten tip. Further, the nitrogen gas produces a tungsten nitride by bonding with tungsten atoms on the tip surface. Since field evaporation intensity of the tungsten nitride is small, only a tip side slightly away from the free end where the nitrogen gas is absorbed is selectively etched. However, the tungsten atoms at the free end of the tungsten tip are not etched, so a tip having a free end sharpened further than a tip obtained by electrolytic polishing is obtained (for example, see Patent Document 1).
The thermal faceting is a method of making a polyhedral structure at the free end of a tip by growing predetermined facets by heating a tip, which has undergone electrolytic polishing, under a oxygen atmosphere (for example, see Patent Document 2).
The remolding is a method of making a facet at the free end of a tip by heating and applying high voltage to a tip, which has undergone electrolytic polishing, under ultra-high vacuum (for example, see Patent Document 3).
Further, as a method of making a tip of which the free end is formed of one atom, there is a method of plating a tungsten or molybdenum tip with gold, platinum, palladium, iridium, rhodium, or alloys of them and then applying electrolytic polishing or heating the tip, thereby making a single atom structure (for example, see Patent Document 4).
Further, there is a scanning ion microscope (also called an FIB apparatus) using helium FIB and equipped with a gas field ion source using a tungsten tip (for example, see Non-Patent Document 1).
Further, there is a scanning ion microscopes (also called an FIB apparatus) using helium FIB and equipped with a gas field ion source using a tungsten tip in which the free end of the tungsten tip discharging ions is made of a terpolymer composed of three tungsten atoms (for example, see Non-Patent Document 2).
Further, in order to make a tip made of iridium having higher chemical resistance than tungsten, where the free end of an iridium thin wire has a pyramidal structure composed of one atom, there is a method of applying heat by supplying oxygen into a vacuum container (thermal faceting) (for example, see Patent Document 2).
Further, there is a monocrystal tip made of <210> iridium and having a free end having a small pyramidal structure composed of one {110} facet and two {311} facets (for example, see Non-Patent Document 3).
Further, there is a sharpened monocrystal tip made of <210> iridium and having a free end that has a small pyramidal structure composed of one {110} facet and two {311} facets by thermal faceting and has an apex made of one atom. There is a case that a gas field ion source using this iridium tip has continuously operated for about 2250 seconds (about 37.5 minutes) (for example, see Non-Patent Document 4).
The free end structure of an iridium monocrystal tip of the related art is shown in FIGS. 14A to 15B. FIGS. 14A and 15A are model diagrams of a pyramidal structure when an iridium tip of the related art is seen in a <210> orientation. FIGS. 14B and 15B are schematic diagram simply showing facets. FIGS. 14A and 14B were made with reference to Non-Patent Document 3 and FIGS. 15A and 15B were made with reference to Non-Patent Document 4.
In FIGS. 14A and 15A, an iridium atom 551 on the uppermost surface (outermost surface) of the facets is shown in a white circle and iridium atoms 41 inside under the uppermost surface are shown in gray circles. One iridium atom 551 (552) is positioned at the apex of the pyramid and iridium atoms 551 (553) on the ridges of the pyramid are given black triangles. In FIGS. 14B and 15B, the pyramids each have ridges 555a, 555b, and 555c formed by three conical surfaces 554a, 554b, and 554c and an apex 556 formed by one iridium atom 551 (552).
In FIGS. 14B and 15B, the conical surface 554a is a {110} facet and the conical surfaces 554b and 554c are {311} facets. That is, by using an iridium thin wire, the free end of a <210> iridium monocrystal tip has a pyramidal structure having one {110} facet and two {311} facet, and having one iridium atom at the apex.
The difference of the atomic arrangement of FIG. 14A and FIG. 15A is that the number of the atoms 551 in the bottom of the conical surface 544a is an odd number or an even number, and it is an even number in FIG. 14A and an odd number in FIG. 15A. Depending on whether the number of iridium atoms 551 in the bottoms is an odd number or an even number, the arrangement of iridium atoms 551 in a second layer and the third layer under the iridium atom 551 (552) at the uppermost layer, that is, the apex of the free end becomes different. The second layer in the atomic arrangement shown in FIG. 14A includes three iridium atoms 551 and the second layer in the atomic arrangement shown in FIG. 15A includes six iridium atoms 551. Further, the iridium atom 551 (552) at the apex of the pyramidal structure shown in FIG. 15A is positioned at the intersection of three ridges 555a, 555b, and 555c. On the contrary, the iridium atom 551 (552) at the apex of the pyramidal structure shown in FIG. 14A is at a position slightly protruding from the intersection of three ridges 555a, 555b, and 555c. 